Method for estimating doppler frequency

ABSTRACT

Disclosed is a method for estimating a Doppler frequency with phase information, thereby estimating a maximum Doppler frequency. The method for estimating the Doppler frequency includes the steps of measuring phases of phase samples from a plurality of slots of a received signal, evaluating a phase difference of the measured phase, and estimating a maximum Doppler frequency according to a mean of at least one phase difference.

[0001] This application claims the benefit of the Korean Application No.P2002-81721 filed on Dec. 12, 2002, which is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a mobile communicationtechnology, and more particularly, to a method for estimating a Dopplerfrequency with phase information, thereby estimating a maximum Dopplerfrequency.

[0004] 2. Discussion of the Related Art

[0005] According as frequency resource becomes important in mobilecommunication technology, adaptive receiver techniques have been paidmuch attention to an efficient usage of the frequency resource and aflexible resource management. There are a lot of factors to optimize aperformance of the adaptive receiver. Above all, to estimate a maximumDoppler frequency is one of the most valuable factors. The maximumDoppler frequency has been estimated according to the variation of themeasured pilot signal strength. However, the method for estimating amaximum Doppler frequency may have errors in case of that the adaptivereceiver has a great Gaussian noise or a very high Doppler frequency.

[0006] In a communication system using a Phase Shift Keying (PSK)modulation of a non-coherent method, a maximum Doppler frequency is alsoestimated based on variations of the received signal strength.Particularly, in case of a communication system using Differential PhaseShift Keying (DPSK) modulation, because a pilot signal is nottransmitted to a receiver, the receiver may estimate a maximum Dopplerfrequency using variations of another received signal such as trafficsignal.

[0007] However, if the aforementioned method for estimating the maximumDoppler frequency with variations of a power for the received signal isapplied to CDMA system using a rapid closed loop power control method,it has problems decreasing an accuracy in the estimating of a maximumDoppler frequency.

SUMMARY OF THE INVENTION

[0008] Accordingly, the present invention is directed to a method forestimating Doppler frequency that substantially obviates one or moreproblems due to limitations and disadvantages of the related art.

[0009] An object of the present invention is to provide to a method forestimating Doppler frequency for improving accuracy in the estimate ofthe maximum Doppler frequency with phase variations (phase differences)of received signals.

[0010] Additional advantages, objects, and features of the inventionwill be set forth in part in the description which follows and in partwill become apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objectives and other advantages of the invention may berealized and attained by the structure particularly pointed out in thewritten description and claims hereof as well as the appended drawings.

[0011] To achieve these objects and other advantages and in accordancewith the purpose of the invention, as embodied and broadly describedherein, a method for estimating a Doppler frequency, comprises the stepsof measuring phases of phase samples from a plurality of slots of areceived signal, evaluating a phase difference of the measured phase,and estimating a Doppler frequency according to a mean of at least onephase difference.

[0012] In another aspect of the present invention, a method forestimating a Doppler frequency, comprises the steps of measuring phasesof phase samples for successive slots of a demodulated pilot signal ateach predetermined period τ, evaluating a phase difference of the phasesmeasured at the period τ, evaluating a mean of an absolute phasedifference, and estimating a maximum Doppler frequency with the mean ofthe absolute phase differences and the period τ.

[0013] In still another aspect of the present invention, a method forestimating a Doppler frequency, comprises the steps of extracting phasesamples for each of successive slots of a phase shift keying (PSK)demodulated signal, compensating the phases of a portion of the phasesamples for each of the successive slots, evaluating a mean phase of thephase samples for each of the successive slots, evaluating respectivedifferences of mean phases for every successive slots, evaluating a meanof absolute values of the differences, and estimating a maximum Dopplerfrequency with the mean of the absolute values and a period τ ofextracting the phase samples.

[0014] It is to be understood that both the foregoing generaldescription and the following detailed description of the presentinvention are exemplary and explanatory and are intended to providefurther explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The accompanying drawings, which are included to provide afurther understanding of the invention and are incorporated in andconstitute a part of this application, illustrate embodiment(s) of theinvention and together with the description serve to explain theprinciple of the invention. In the drawings:

[0016]FIG. 1 is a diagram illustrating a method for estimating a maximumDoppler frequency according to the first preferred embodiment of thepresent invention;

[0017]FIG. 2 is a diagram illustrating a method for estimating a maximumDoppler frequency according to the second preferred embodiment of thepresent invention;

[0018]FIG. 3 illustrates standard constellation of a transmitted DPSKmodulated signal; and

[0019]FIG. 4 illustrates standard constellations of a received DPSKdemodulated signals for at least two successive slots.

DETAILED DESCRIPTION OF THE INVENTION

[0020] Reference will now be made in detail to the preferred embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings. Even though the present invention is illustratedwith reference to the preferred embodiments, the present invention isnot limited to the following preferred embodiments. That is, the presentinvention may include variations if they come within the scope of themain technology of the present invention.

[0021] In a method for estimating a maximum Doppler frequency accordingto a first preferred embodiment of the present invention, a phasedifference of phases measured at each predetermined period τ iscalculated, and a mean of phase differences calculated from the measuredphases is calculated. And then, a maximum Doppler frequency is estimatedfrom the calculated mean phase difference.

[0022] Especially, the present invention may be adapted to acommunication system which does not transmit a pilot signal. That is, ina communication system transmitting a pilot signal, a maximum Dopplerfrequency is estimated based on phase variations in slots of the pilotsignal. Meanwhile, a communication system, in which a pilot signal isnot transmitted, uses phase differences in slots of a demodulatedtraffic signal to estimate the maximum Doppler frequency.

[0023] A method for estimating a maximum Doppler frequency according toa first preferred embodiment of the present invention includes followingfour steps.

[0024] 1. Measure phases ( . . . , φ_(i−1), φ_(i), . . . ) of phasesamples from a plurality of slots in a received signal at eachpredetermined period τ. At this time, the received signal may be a pilotsignal demodulated according to a coherent method, and a PSK demodulatedsignal (DPSK demodulated signal or BPSK demodulated signal) according toa non-coherent method.

[0025] 2. Evaluate a phase difference φ_(i−1)−φ_(i) between the phasesamples measured for every at least two successive slots. And then, anabsolute phase difference |φ_(i)−φ_(i−1)| is evaluated.

[0026] 3. Evaluate a mean Z of absolute phase differences as follows.${\hat{f}}_{D} = \frac{Z}{\sqrt{2}{\pi\tau}}$

[0027] 4. Apply the mean Z of the absolute phase differences to thefollowing equation$Z = \left. {\frac{1}{N}\sum\limits_{n = 0}^{N}}\quad \middle| {\varphi_{n} - \varphi_{n - 1}} \right|$

[0028] where $\frac{1}{\sqrt{2}}$

[0029] is a constant value used for estimating a maximum Dopplerfrequency in urban environments, and the constant value may vary basedon a place of estimating a Doppler frequency. Therefore, the maximumDoppler frequency (f_(D)) is estimated.

[0030]FIG. 1 is a diagram illustrating a method for estimating a maximumDoppler frequency according to the first preferred embodiment of thepresent invention. That is, FIG. 1 illustrates the method for estimatingthe maximum Doppler frequency with a phase variation of a pilot signal.

[0031] Referring to FIG. 1, phases ( . . . , φ_(i−1), φ_(i), . . . ) ofphase samples are measured from a demodulated pilot signal at eachpredetermined period τ (S1). Subsequently, a phase difference of the twosuccessive phase samples is evaluated (S2). The phase difference (ζ_(i))of the at least two successive phase samples is expressed as followingEquation 1.

ζ_(i)=φ_(i)−φ_(i−1)  [Equation 1]

[0032] When ζ_(i) is a phase difference φ₂−φ₁ of the respective phasesφ₂=φ(t+τ) and φ₁=(t), ζ satisfying −π<ζ<π, the probability densityfunction P(ζ) of ζ is expressed as following Equation 2. At this time,the following Equation 2 is an example of a probability density functionof ζ in urban environments. $\begin{matrix}{{P(\xi)} = {\frac{1 - \lambda^{2}}{2\pi} \cdot \frac{\sqrt{1 - {\lambda^{2}\cos^{2}\xi}} + {{\lambda cos\xi cos}^{- 1}\left( {- {\lambda cos\xi}} \right)}}{\left( {1 - {\lambda^{2}\cos^{2}\xi}} \right)^{3/2}}}} & \left\lbrack {{Equation}\quad 2} \right\rbrack\end{matrix}$

[0033] In the above Equation 2, ζ=J₀(2πf_(Dτ)). At this time, f_(D) is amaximum Doppler frequency.

[0034] Next, a mean of absolute phase differences evaluated at eachpredetermined period τ is evaluated (S3). Meanwhile, a mean E(|ζ|) of anabsolute phase difference of ζ denoting a phase difference of the phasesamples can be expressed as following Equation 3. $\begin{matrix}{\left. {{E\left( |\xi| \right)}==\int_{0}^{\pi}} \middle| \frac{\xi}{\pi} \middle| {{P(\xi)}\quad {\xi}} \right. = {\int_{0}^{\pi}{\frac{\xi}{\pi}{\left( {1 - \lambda^{2}} \right) \cdot \frac{1 - \lambda^{2}}{2\pi} \cdot \frac{\sqrt{1 - {\lambda^{2}\cos^{2}\xi}} + {{\lambda cos\xi cos}^{- 1}\left( {- {\lambda cos\xi}} \right)}}{\left( {1 - {\lambda^{2}\cos^{2}\xi}} \right)^{3/2}}}\quad {\xi}}}} & \left\lbrack {{Equation}\quad 3} \right\rbrack\end{matrix}$

[0035] In Equation 3, the mean E(|ζ|) of the absolute phase differencesof ζ denoting the phase difference of the phase samples is directlyrelated to a maximum Doppler frequency f_(D).

[0036] A mean of absolute phase differences evaluated from ‘N’ phasesamples is evaluated as following Equation 4. $\begin{matrix}{Z = \left. {\frac{1}{N}\sum\limits_{n = 0}^{N}}\quad \middle| {\varphi_{n} - \varphi_{n - 1}} \middle| {{E\left( |\xi| \right)} \approx Z_{N\rightarrow\infty}} \right.} & \left\lbrack {{Equation}\quad 4} \right\rbrack\end{matrix}$

[0037] In the above Equation 4, as ‘N’ is larger, ‘Z’ converges toE(|ζ|). That is, ‘Z’ is the approximation of E(|ζ|) evaluated at eachpredetermined period τ.

[0038] According to Equation 3 and 4, a maximum Doppler frequency f_(D)is related with a mean of an absolute phase difference ‘Z’ as shownEquation 5. $\begin{matrix}{{\hat{f}}_{D} = {K\frac{Z}{\pi\tau}}} & \left\lbrack {{Equation}\quad 5} \right\rbrack\end{matrix}$

[0039] In Equation 5, the maximum Doppler frequency is estimated (S4).Also, a constant value K varies based on a radio environment condition.For example, K is $\frac{1}{\sqrt{2}}$

[0040] in the urban environments. At this time, a period τ satisfyingf_(Dτ)<0.4 (where $K = \frac{1}{\sqrt{2}}$

[0041] ) may be marginally set to estimate a maximum Doppler frequencyin a high Doppler frequency region. In this state, ‘Z’ obtains anaccuracy of the approximation when satisfying f_(Dτ)<0.4.

[0042] The first preferred embodiment of the present invention using aphase difference of a demodulated pilot signal does not have anyperformance degradation in a communication system using the closed looppower control like CDMA system. Accordingly, if the first preferredembodiment of the present invention is applied to the CDMA system, anaccuracy in estimating of a maximum Doppler frequency is improved.

[0043] However, in a communication system using a PSKmodulation/demodulation of the non-coherent method, a pilot signal isnot transmitted. A method for estimating a maximum Doppler frequency inthe communication system that does not transmit the pilot signal will bedescribed as follows.

[0044]FIG. 2 is a diagram illustrating a method for estimating a maximumDoppler frequency according to the second preferred embodiment of thepresent invention. FIG. 2 illustrates a case applied to a communicationsystem using a PSK modulation/demodulation of the non-coherent method,especially in the communication system using a DPSKmodulation/demodulation. Thus, a maximum Doppler frequency is estimatedwith a mean of phase differences of a PSK demodulated signal. Also, amethod for estimating a maximum Doppler frequency according to thesecond preferred embodiment of the present invention may be applied to acommunication system using a BPSK modulation/demodulation.

[0045] An estimating procedure of FIG. 2 will be explained withreference to FIG. 3 and FIG. 4. FIG. 3 illustrates a standardconstellation of a transmitted DPSK modulated signal, and FIG. 4illustrates a standard constellation of a received DPSK demodulatedsignal for at least two successive slots.

[0046] When a DPSK modulated signal is “0”, a binary signal value, theDPSK modulated signal is transmitted without a phase shifting.Meanwhile, when a DPSK modulated signal is “1”, the DPSK modulatedsignal may have a phase shifting of 180 degrees, and then transmitted.

[0047] A constellation of a received DPSK demodulated signal is shown inFIG. 3, and a constellation of the received DPSK demodulated signal forat least two successive slots are shown in FIG. 4. In FIG. 4, therespective constellations means samples of a received signal.

[0048] A method for estimating a maximum Doppler frequency according tothe second preferred embodiment of the present invention will bedescribed with reference to FIG. 2.

[0049] First, a phase difference is evaluated for being applied to theaforementioned Equations 4 and 5. In fact, it is impossible to evaluatean absolute phase difference |φ_(n)−φ_(n−1)| applied to Equation 4 in acommunication system using a DPSK modulation/demodulation of thenon-coherent method. Thus, the absolute phase difference |φ_(n)−φ_(n−1)|applied to Equation 4 is evaluated through the following first to fifthsteps, and a maximum Doppler frequency is estimated in the sixth step atthe same way as the first embodiment of the present invention.

[0050] STEP 1: In FIG. 4 illustrating a constellation of the phasesamples extracted for the ‘i’th slot, most of the phase samples aredistributed in two quadrants. That is, the phase samples distributed inone quadrant are transmitted without the phase shifting, and the phasesamples distributed in the other quadrant are shifted at 180 degrees,and then transmitted. Thus, in a method for estimating a maximum Dopplerfrequency according to second preferred embodiment of the presentinvention, a dominant quadrant having many phase samples is firstlychecked. At this time, the dominant quadrant may be a quadrant havingthe phase samples transmitted without the phase shifting, or not.

[0051] STEP 2: The phase samples are compensated for phases with a phaseoffset of 180 degrees so that all of phase samples may be located in thedominant quadrant of FIG. 4. Preferably, in the present invention, thephase samples transmitted from a transmitter without the phase shiftingare distributed in the dominant quadrant, and the phase samplesdistributed in the other quadrant and transmitted from the transmitterwith a phase shifting are compensated with a phase offset of 180 degrees(for compensating the phase of the DPSK modulated signal).

[0052] STEP 3: After compensating the phases samples with the phaseoffset of 180 degrees, a mean phase of the phase samples extracted forthe ‘i’th slot is evaluated. That is, a mean phase {circumflex over(θ)}_(i) of the phase samples for the ‘i’th slot is evaluated.

[0053] STEP 4: For the next slot, (i+1)th slot, aforementioned first tothird steps are performed to evaluate a mean phase {circumflex over(θ)}_(i+1) of phase samples extracted for the ‘i+1’th slot.

[0054] STEP 5: A phase difference Δ{circumflex over (θ)}_(i) isevaluated between the mean phase {circumflex over (θ)}_(i) for the ‘i’thslot and the mean phase {circumflex over (θ)}_(i+1) for the ‘i+1’th slotaccording to following Equations 6 and 7. $\begin{matrix}{{\Delta {\hat{\theta}}_{i}} = \begin{bmatrix}{{k_{i} \cdot \left( {{\hat{\theta}}_{i + 1} - {\hat{\theta}}_{i}} \right)},\quad \left. \left. \text{for}\quad||{{\hat{\theta}}_{i + 1} - {\hat{\theta}}_{i}} \right. \middle| {< \left| \left. {{\hat{\theta}}_{i + 1} - {\left( {{\hat{\theta}}_{i} + \pi} \right)\text{mod}\left( {2\pi} \right)}} \right.|| \right.} \right.} \\{{{k_{i} \cdot {\hat{\theta}}_{i + 1}} - {\left( {{\hat{\theta}}_{i} + \pi} \right)\text{mod}\left( {2\pi} \right)}},\quad \left. \left. \text{for}\quad||{{\hat{\theta}}_{i + 1} - {\hat{\theta}}_{i}} \right. \middle| {> \left| \left. {{\hat{\theta}}_{i + 1} - {\left( {{\hat{\theta}}_{i} - \pi} \right)\text{mod}\left( {2\pi} \right)}} \right.|| \right.} \right.}\end{bmatrix}} & \left\lbrack {{Equation}\quad 6} \right\rbrack \\{k_{i} = \begin{matrix}{1,\quad {{\text{if}\quad \alpha_{i}} > \alpha_{thresh}}} \\{0,\quad {{\text{if}\quad \alpha_{i}} < \alpha_{thresh}}}\end{matrix}} & \left\lbrack {{Equation}\quad 7} \right\rbrack\end{matrix}$

[0055] In Equations 6 and 7, K_(i) is a reliability factor, and α_(i) isa mean power the phase samples extracted for the ‘i’th slot. In Equation7, a received signal having a power more than a predetermined thresholdvalue (α_(thresh)) is used for estimating a maximum Doppler frequency.

[0056] In equation 6 of the present invention, a modular calculation isused with regard to selecting a dominant quadrant in which phase sampleshaving shifted phases are distributed. That is, equation 6 takes intoaccount of compensating for the phase samples with a phase offset of 180degrees, wherein the phase samples are transmitted from a transmitterwithout a phase shifting.

[0057] In the second preferred embodiment of the present invention, theabove first to fifth steps are repeated ‘N’ times at each predeterminedperiod τ.

[0058] STEP 6: The phase differences calculated ‘N’ times are applied toEquation 4 to evaluate a mean of absolute phase differences calculated‘N’ times. Thus, the evaluated approximation Z is applied to Equation 5,whereby a maximum Doppler frequency is estimated.

[0059] In the second preferred embodiment, a phase difference|{circumflex over (θ)}_(i+1)−{circumflex over (θ)}_(i)| of at least twosuccessive slots is calculated from the respective mean phases{circumflex over (θ)}_(i) and {circumflex over (θ)}_(i+1) of the phasesamples evaluated according to the third to fourth steps. Next, thephase difference is applied to Equation 4 to evaluate a mean of absolutephase differences, and the evaluated mean of the absolute phasedifferences is applied to Equation 5 to estimate a maximum Dopplerfrequency.

[0060] However, in the present invention, the calculated phasedifference is decreased by the reliability factor k_(i). This reason isthat a phase difference evaluated from a received signal of a low powermay generate errors in estimating a maximum Doppler frequency.

[0061] In the second preferred embodiment of the present invention, itis possible to extract phase information of a radio channel from a DPSKdemodulated signal of the non-coherent method. Also, a maximum Dopplerfrequency is estimated by the extracted phase information.

[0062] As mentioned above, a method for estimating a maximum Dopplerfrequency according to preferred embodiments of the present inventionhas the following advantages.

[0063] In a communication system having a pilot signal, it is possibleto improve accuracy in estimating a maximum Doppler frequency with phasevariations for a plurality of slots of the pilot signal. In case of acommunication system in which a pilot signal is not transmitted(especially, the system using a phase modulation/demodulation of thenon-coherent method), a maximum Doppler frequency is estimated with aphase difference for at least two successive slots of a demodulatedtraffic signal with accuracy.

[0064] Especially, even if the method estimating a maximum Dopplerfrequency with the statistics of phase differences is applied to a CDMAsystem using a rapidly closed loop power control, accuracy is notimpaired in estimating a maximum Doppler frequency because the phaseinformation of the received signals is not influenced by the closed looppower control.

[0065] It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present invention. Thus,it is intended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A method for estimating a Doppler frequency,comprising: (a) measuring phases of phase samples from a plurality ofslots of a received signal; (b) evaluating a phase difference of themeasured phase; and (c) estimating a Doppler frequency according to amean of at least one phase difference.
 2. The method of claim 1, whereinthe received signal is a pilot signal in the (a) step, thereby measuringthe phases of the phase samples for successive slots of the pilot signalat each predetermined period.
 3. The method of claim 1, wherein thereceived signal is a phase shift keying (PSK) modulated signal accordingto a non-coherent method in the (a) step, thereby measuring the phasesof the phase samples for successive slots of the PSK modulated signal ateach predetermined period.
 4. The method of claim 1, further comprisingthe step of evaluating an absolute phase difference of the measuredphase samples.
 5. The method of claim 1, the (c) step comprising thesteps of: (d) evaluating a mean of absolute phase differences calculatedat each predetermined period; and (e) estimating a maximum Dopplerfrequency with the evaluated mean of the absolute phase differences andthe period (τ).
 6. A method for estimating a Doppler frequency,comprising: (a) measuring phases of phase samples for successive slotsof a demodulated pilot signal at each predetermined period τ; (b)evaluating a phase difference of the phases measured at the period τ;(c) evaluating a mean of an absolute phase difference; and (d)estimating a maximum Doppler frequency with the mean of the absolutephase difference and the period τ.
 7. The method of claim 6, wherein amean of absolute phase differences for ‘N’ phase samples is evaluated inan approximation as follows in the (c) step.$Z = {\frac{1}{N}{\sum\limits_{n = 0}^{N}\quad {{\varphi_{n} - \varphi_{n - 1}}}}}$


8. The method of claim 6, wherein, in the (d) step, a mean Z of absolutephase differences for ‘N’ phase samples is evaluated according tofollowing equation,$Z = {\frac{1}{N}{\sum\limits_{n = 0}^{N}\quad {{\varphi_{n} - \varphi_{n - 1}}}}}$

whereby the maximum Doppler frequency ({circumflex over (f)}_(D)) isestimated by $\hat{f_{D}} = {K\frac{Z}{\pi \quad \tau}}$

where K is a constant value according to a radio environment condition.9. A method for estimating a Doppler frequency, comprising: (a)extracting phase samples for each of successive slots of a phase shiftkeying (PSK) demodulated signal; (b) compensating the phases of aportion of the phase samples for each of the successive slots; (c)evaluating a mean phase of the phase samples for each of the successiveslots; (d) evaluating respective differences of mean phases for everysuccessive slots; (e) evaluating a mean of absolute values of thedifferences; and (f) estimating a maximum Doppler frequency with themean of the absolute values and a period τ of extracting the phasesamples.
 10. The method of claim 9, wherein the PSK demodulated signalis modulated by one of DPSK and BPSK modulation methods.
 11. The methodof claim 9, wherein the (b) step comprising: (g) defining one quadrantas a dominant quadrant, the dominant quadrant in which lots of phasesamples are distributed; and (h) compensating phases of phase sampleswhich are not distributed in the main quadrant so that all of phasesamples may be distributed in the main quadrant.
 12. The method of claim11, wherein, if the PSK demodulated signal is a DPSK demodulated signalin the step (h), phases of the phase samples which are not distributedin the main quadrant are compensated at 180 degrees.
 13. The method ofclaim 9, wherein a difference of the mean phases is evaluated for everysuccessive slots of the PSK demodulated signal having a power more thana predetermined threshold value (α_(thresh)) in the (d) step.
 14. Themethod of claim 9, wherein, in the (f) step, a mean of the absolutephase differences for ‘N’ phase samples is evaluated according tofollowing equation,$Z = {\frac{1}{N}{\sum\limits_{n = 0}^{N}\quad {{\varphi_{n} - \varphi_{n - 1}}}}}$

whereby the maximum Doppler frequency ({circumflex over (f)}_(D)) isestimated by $\hat{f_{D}} = {K\frac{Z}{\pi \quad \tau}}$

where K is a constant value varying according to a radio environmentcondition.